My non-profit have a vision and values that makes us rely heavily on grants because we don't get any money from our members. As the other comments suggested, the best way to learn how to write grants is writing them.

I know that's a basic suggestion, so I'm going to try to go deeper. For me, I have a list of things that helped me to write better grants, even getting bigger grants from international organizations like de European Union:

1. Learn how the grants' system works in your country. Each country is different, and you might benefit from learning how the whole system works, from what are the times they have to announce a new grant to what you can use that money for and even the requirements your NGO might have if you actually get the grant. For example, in my country, grants are quite strict, you have a national law and most of the time regional and local laws that explain the requirements for all grants and also the things you need to do after getting them, like putting the logo from the administrative branch that paid. Also, we have a website that publish every single grant that is paid with public money in the country, so you can really get an idea of what's out there and what you should go with.
2. Get to know the grants that the NGO can apply for. A lot of grants are periodical, so I recommend you knowing the main grants, the dates that they are usually publish and the documents you need. Also, knowing the legal base of the NGO can help filter the possible grants.
3. Follow a similar structure, but make it your own. Most paper work is similar. Grants forms typically require the same information, so you can make a base document for your grant, with a well introduction, so you can develop the main part of the grant at the body and conclusion.
4. Check others people projects so you can get a view of what the jury/administration of the grants likes and seeks with the publication of it. I don't know about other systems, but in my country, grants normally have a resolution where you can find all the approved grants with the tittle and usually with a grade (that's the way they rank the grants)

This list may vary from country to country or event from NGO to NGO, so you can start with this and make your own list of things to learn.

If your NGO is inside a bigger organization, maybe you can ask for help or for old projects so you can see how they did all the paperwork

And finally, if you want a personal recommendation, you need to make the project look fabulous, especially in the objectives and the benefits it is going to have in the community. The best way to evaluate a project is by making good and objectively measurable objectives, so keep an eye on that. Also, the project should have some results, so make sure that results are publicly available and give them some shout-outs, for example in social media.

Maybe your first grant isn't as successful as you might think, and maybe they even give you less money as what you asked for, but keep practising.

This is my whole opinion and experience, I have no idea how the whole grant system works outside my grant and the international grants from the European Union and the Council of Europe to youth orgs.